The present invention is concerned with a device for writing to and/or reading from optical recording media having different structures such as, for example, recording media of high storage density in the form of a videodisk and recording media of low storage density corresponding to a known CD.
A device of this type is disclosed, for example, in DE 43 08 531. In the case of this known device, a defocusing means is introduced into the beam path in order deliberately to defocus the light which is incident on a detector device and to produce a scanning spot size which is adapted to the type of optical recording medium. However, this presupposes that the recording media have a structure that corresponds right down to the pit size.
However, optical recording media such as, for example, compact discs (CD) or digital videodisks (DVD) have different substrate thicknesses. Reading and/or writing therefore require different numerical apertures, which correspondingly require different objective lenses and different aperture angles.
The object of the invention, therefore, is to propose a device for writing to and/or reading from optical recording media having different structures which also makes it possible to write to and/or read from optical recording media having different substrate thicknesses using just one objective lens.
This object is achieved by means of the measures specified in the independent claims.
The invention provides for the objective lens to have a diaphragm of variable diameter. The functions of diaphragm and objective lens are preferably integrated in one component, but it also lies within the scope of the invention to design the objective lens and the diaphragm as components which are independent but combined with one another. The diaphragm used here may be, for example, a mechanical diaphragm which can be set to different diameters, or different diaphragms of varying diameter which can be introduced into the beam path, or else other suitable diaphragms. One advantage of the arrangement according to the invention resides in the fact that the lens is adapted, that is to say corrected, to that optical recording medium whose numerical aperture is large. The diaphragm having the largest possible diameter is used in this case. The arrangement works with a smaller diaphragm opening for an optical recording medium which requires a smaller numerical aperture. In this way, not only is the numerical aperture set correctly, but also the negative effects of the lens not being optimally corrected to this optical recording medium are mitigated. The wavefront perturbations are reduced by the smaller diaphragm diameter.
The objective lens must be corrected to the wavefront perturbation caused by the substrate thickness of the optical recording medium, a corresponding numerical aperture must be set. This is not the case with the device disclosed in the prior art. The single objective lens used there cannot be optimally corrected to any of the possible substrate thicknesses, the correction of the lens necessarily constitutes a compromise. This has the disadvantage that the optical detector arrangement reacts extremely sensitively to tilting of the optical recording medium and to local thickness changes in the transparent protective layer thereof. Moreover, the spot formation on the detector is not uniform since the wavefronts are perturbed, which may lead to consequential errors.
The diaphragm of variable diameter can advantageously be formed by a liquid-crystal element which consists, for example, of one or more annular regions which can be switched to allow passage or effect blocking, and consequently determine, in combination with one another, the size of the diaphragm opening.
The diaphragm can advantageously be formed by a polarization filter, which acts as a diaphragm in interaction with suitable polarization means, which determine the polarization of the light beam. In general, the light beam is generated by a laser diode which emits largely polarized light. An additional optical element for changing the polarization can be introduced into the beam path. The polarization filter is designed in such a way that it represents different diaphragm diameters for light of varying polarization. In the simplest case, the polarization filter has a circular hole which forms the diaphragm opening in the blocking direction, whereas in the passage direction the diaphragm opening is formed by the boundary of the objective lens.
If the polarization filter is directly coupled to the objective lens, then the diaphragm moves together with the objective lens in the event of the objective lens being displaced along the optical axis, as a result of which the numerical aperture determined by the diaphragm size remains constant even during this tracking movement. A tracking movement of this type occurs, for example, in order to be able to compensate for the so-called vertical wobble of the optical recording medium. In this case, the polarization filter can be coupled mechanically to the objective lens or advantageously be directly fitted thereon, for example bonded on.
A particularly simple option for rotating the polarization direction of linearly polarized light consists in introducing a so-called half-wave plate, also called xcex/2 plate, into the beam path. If an additional diverging lens is introduced into the beam path, then this can be used for wavefront correction of the objective lens to a different substrate thickness. The objective lens is generally corrected to a specific substrate thickness, adaptation to the different substrate thickness taking place via the diverging lens. At the same time, the distance of the focal point from the objective lens is increased. Exact adjustment of the diverging lens in the beam path is unnecessary when it is combined with a small diaphragm diameter. The diverging lens used here may be a conventional diverging lens or, alternatively, any other suitable optical element which serves for wavefront correction, for example a holographic element.
The diverging lens is preferably coupled to a further optical element, for example to the polarization means, the diaphragm or the like. If the diverging lens and the further optical element are components to be brought simultaneously into the beam path, then they can be optimally coupled together in this way, with the result that only a single adjustment for both components is necessary. However, it may also be advantageous to couple the diverging lens to a component which should not be in the beam path at the same time, the coupling then ensuring that the two components are not located in the beam path at the same time.
The device according to the invention is particularly suitable for use with optical recording media which contain digitally recorded information.
Further advantageous requirements of the invention can be gathered from the following description of an exemplary embodiment with reference to the figures.